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Date:   Tue, 11 Feb 2020 12:49:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook
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On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote:
> +	if (!rcu_watching) {						\
> +		/* Can not use RCU if rcu is not watching and in NMI */	\
> +		if (in_nmi())						\
> +			return;						\
> +		rcu_irq_enter_irqson();					\
> +	}								\

I saw the same weirdness in __trace_stack(), and I'm confused by it.

How can we ever get to: in_nmi() && !rcu_watching() ? That should be a
BUG.  In particular, nmi_enter() has rcu_nmi_enter().

Paul, can that really happen?